Add needs-run-enabled directive for should-fail tests

I was wary of doing any automatic disabling here, since should-fail
is how we test compiletest itself.

src/test/debuginfo/should-fail.rs

@@ -2,6 +2,7 @@
 
 // == Test [gdb|lldb]-[command|check] are parsed correctly ===
 // should-fail
+// needs-run-enabled
 // compile-flags:-g
 
 // === GDB TESTS ===================================================================================

src/test/ui/meta/revision-bad.rs

@@ -4,6 +4,7 @@
 // run-fail
 // revisions: foo bar
 // should-fail
+// needs-run-enabled
 //[foo] error-pattern:bar
 //[bar] error-pattern:foo
 

src/tools/compiletest/src/common.rs

@@ -351,6 +351,15 @@ pub struct Config {
     pub npm: Option<String>,
 }
 
+impl Config {
+    pub fn run_enabled(&self) -> bool {
+        self.run.unwrap_or_else(|| {
+            // Auto-detect whether to run based on the platform.
+            !self.target.ends_with("-fuchsia")
+        })
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct TestPaths {
     pub file: PathBuf,         // e.g., compile-test/foo/bar/baz.rs

src/tools/compiletest/src/header.rs

@@ -85,6 +85,10 @@ impl EarlyProps {
                     props.ignore = true;
                 }
 
+                if !config.run_enabled() && config.parse_name_directive(ln, "needs-run-enabled") {
+                    props.ignore = true;
+                }
+
                 if !rustc_has_sanitizer_support
                     && config.parse_name_directive(ln, "needs-sanitizer-support")
                 {

src/tools/compiletest/src/runtest.rs

@@ -369,11 +369,7 @@ impl<'test> TestCx<'test> {
     }
 
     fn run_if_enabled(&self) -> WillExecute {
-        let enabled = self.config.run.unwrap_or_else(|| {
-            // Auto-detect whether to run based on the platform.
-            !self.config.target.ends_with("-fuchsia")
-        });
-        if enabled { WillExecute::Yes } else { WillExecute::Disabled }
+        if self.config.run_enabled() { WillExecute::Yes } else { WillExecute::Disabled }
     }
 
     fn should_run_successfully(&self, pm: Option<PassMode>) -> bool {